This invention relates to a light-polarizing material and to a method for preparing a light-polarizing material. More particularly, it relates to a simple and effective method for preparing a light polarizer which exhibits desirable light-polarizing properties and good mechanical strength.
The production of linear light polarizers has been well described in the art. Linear light polarizers, in general, owe their properties of selectively passing radiation vibrating along a given electromagnetic radiation vector (and absorbing electromagnetic radiation vibrating along a second given electromagnetic radiation vector) to the anisotropic character of the transmitting medium. Dichroic polarizers are linear polarizers of the absorptive variety and owe their light-polarizing capabilities to the vectorial anisotropy of their absorption of incident lightwaves. Light entering a dichroic medium encounters two different absorption coefficients-- one low and one high. The emerging light vibrates predominantly in the direction of low absorption.
The most widely used type of synthetic polarizer (and a type of polarizer to which the present invention is directed) is the polyvinyl alcohol-iodine complex polarizer. It consists of linear polyiodide contained within a polyvinyl alcohol matrix. By orienting the polyvinyl alcohol matrix unidirectionally, the transition moments of the absorbers are also so oriented and the material becomes visibly dichroic. The manufacture of iodine-stained dichroic light-polarizing elements has been described in numerous patents, including U.S. Pat. No. 2,237,567 (issued Apr. 8, 1941 to E. H. Land). Various improvements in methods for the production of light polarizers and in the light-polarizing properties thereof have also been described, for example, in U.S. Pat. No. 2,375,963 (issued May 15, 1945 to Alexander Thomas); in U.S. Reissue Pat. No. 23,297 (reissued Nov. 28, 1950 to M. Hyman, Jr., et al.); and in U.S. Pat. No. 4,166,871 (issued Sept. 4, 1979 to N. W. Schuler).
In the production of polarizers of the aforementioned type, it is common to first cast a sheet or film of polyvinyl alcohol (PVA) from an aqueous solution of the material and to heat the dried cast sheet to a temperature at which it can be extended by uniaxial stretching, to so orient the PVA as to permit orientation of the light-absorbing species to be subsequently incorporated therein. Typically, the PVA sheet will be uniaxially stretched to between 3.5 and 4 times its length. Inasmuch as the oriented (stretched) sheet assumes a somewhat fibrous nature and a diminished tensile strength in the direction transverse to the direction of stretch, it is common to bond the stretched PVA sheet to a sheet of supporting material. The supporting sheet is generally a transparent sheet of suitable isotropic material which provides dimensional stability for the PVA sheet and which serves as a cover or protective element for the completed polarizer. Cellulosic materials such as cellulose acetate butyrate, cellulose acetate and cellulose triacetate are suitable for these purposes. The supported sheet of oriented PVA can then be stained with iodine solution or otherwise rendered visibly dichroic by the use of methods known in the art.
The use of a support material for the oriented (stretched) PVA sheet material necessitates that each of the PVA and support sheets be first separately manufactured and that the resulting sheet elements be bonded or laminated in a separate laminating operation, using a suitable adhesive bonding material. It has been disclosed in U.S. Pat. No. 4,659,523 (issued Apr. 21, 1987 to J. H. Rogers, et al.) that an iodine stainable polyester polarizer film can be provided by a process involving the uniaxial stretching of a melted and extruded polyester web carrying an anchor coating and a coating of PVA coated from a dispersed aqueous composition of PVA. Such a process obviates the separate production of a sheet of PVA which would then need to be laminated to the support sheet material.
A light polarizer prepared by a method which involves the application of a stainable coating of PVA (from an aqueous solution of PVA) onto a support sheet prior to simultaneous uniaxial orientation (stretching) of the PVA and support sheet, has been found to produce a light polarizer which is characterized by poor physical properties. While such a polarizer exhibits satisfactory optical (light-polarizing) properties, the plastic light polarizer is readily fractured upon attempts to bend, fold or crease the polarizer material. Fracturing is observed along the direction of stretching, effectively negating the use of the polarizer where rough handling or product abuse is likely to be encountered.